Not necessarily. It's a bit complex, but the fact that Fathom ingests personal data at all likely means that they must still be disclosed by whoever is using Fathom's code.
On the other hand, if Fathom were able to push data ingestion into the first party's infrastructure so that only aggregated data hits Fathom's own infrastructure, I'm pretty sure that would put Fathom in the clear due to the fact that they are no longer processing or storing personal data.
To Fathom's credit, I am a big fan regardless of the steps they seem to be taking with anonymization, and I will consider using them instead of GA for future projects.